Surface proton excess

The powdered carbon samples were titrated under carefully controlled conditions in a cycle of forward and backward titrations to test the reversibility of surface protolytic processes. The range of the titration was limited between pH 3 and 10. The proton-binding isotherms, i.e. the pH-dependence of the net proton surface excess amount (An = - n jj-, mmol/g)... 

Comparison of the proton-binding isotherms of treated samples with that of the untreated carbon reveals a pronounced difference, probably owing to the complete change of surface functional groups. However, the presence of residual acidic impurities cannot be excluded. The positive values of the net proton surface excess amount over the range of pH studied here indicate a significant accumulation of protons on the surface of the... 

The net proton surface excess amount (Ah h/oh, moles per gram) is defined as the difference of H ( "+) and OH ( oh ) surface excess amounts related to umt mass of solid, = n + — n jj. The surface excess amount of any solute, like H and OH here, can be determined directly from the initial and equilibrium concentration of solute for adsorption from dilute solution [23]. The values of and /Iqh calculated at each point of the titration from tne electrode output using the actual activity coefficient from the slope of H /OH" activity versus concentration straight lines for background electrolyte titration. 

Fig. 3 Experimental points of net proton surface excess amounts from the reversible backward titration cycles of sodium montmoril-lonite at different NaCl concentrations. The different lines represent the results of numerical fitting (FITEQL [28]) using the diffuse-double-layer option of the surface complexation model assuming reactions of and Na" ions with permanently charged ion-exchange sites in parallel with protonation/deprotonation reactions on amphoteric edge sites...

Fig. 1 Proton surface excess, x = Z — xN, as a function of mass number. Nuclide periodicity predicts maximal surface spin to occur in the regions as marked, in general agreement with the measured spin and elemental superconductivity of odd mass number nuclides...

It follows from the considerations outlined in the previous section that the charge-determining species on the AI2O3 surface in acidic media are the protons. Denoting the surface excess concentration of protons bound to the surface by F +, the charge balance characterizing the double layer as a whole should be... 

Both r + and FNa+ are negative surface excesses in the case of protonated i.e., positively charged surfaces. 

On the other hand, the changes caused by specific adsorption of anions in the solution side of the double layer can also be followed through the measurement of TCO4 adsorption. Specifically adsorbed anions depending on the extent of their adsorption compensate the positive charge of a protonated surface therefore, the negative charge on the solution side, i.e., the surface excess concentration of nonspecifically adsorbed anions, should decrease. This effect is well demonstrated by the curve presented in Fig. 9. 

The chloride, bromide, iodide, and some other anions offer the advantage of being surface active, too. Their surface excess at the interface can be considered as one of the plates of the electric double layer. And the diffuse part of the double electric layer can be considered as the second plate, which consists of hydrated protons and salt cations. 

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